I would ditch the UMS electronics and use a Battleswitch to turn the pump on
http://www.dimensionengineering.com/...ch-diagram.jpg
You will need a non return valve just before it feeds the valve.
I am now using the
GEMS PS61-19-4MGZ-B-SP pressure switch to control the hydraulic pressure generated. This is wired into the negative wire of the servo line to the Battleswitch.
You need to check the pressure it switches at before installation. Mine came at the max setting 275 psi so I reduced it to around 150 psi. 'You will be fishing in the dark' if you are not checking the pressure of the switch point.
With your UMS system you need to fit a gauge to check the pressure.
I tried the UMS pump and frankly it was nowhere near as good as a standard turbine kero pump for an engine of 100n or more.
I would be amazed if the retracts failed to work at all with hydraulics. You will need to bleed the air from the lines to get it to work well.
John